1. Field of the Invention
This invention relates to an engine induction system including an intake passage provided with a diffuser, a detector for detecting an amount of intake air, a throttle valve, and a bypass pipe having a duty controlled control valve disposed therein.
2. Discussion of the Related Art
There has been known an induction system including an intake passage provided with an air cleaner, an airflow meter for detecting an amount of intake air, and a throttle valve, and having a resonance chamber connected thereto so as to reduce noises generated in taking the intake air into an engine (for example, refer to Japanese Unexamined Utility Model Publication No. 63-141832). In the system of this type, the resonance chamber or the like is provided so as to obtain a function such as a muffling effect of deadening the pulsation according to an intake cycle in a specific engine speed region due to the resonance.
There has been also known an induction system in which an ISC pipe bypassing a throttle valve is provided and an ISC valve is disposed in the ISC pipe for an idle speed control. The ISC valve is duty controlled to control a flow rate of intake air in the ISC pipe. The induction system of this type suffers from the following problem since no consideration has been conventionally made on the influence of the vibrations generated when the ISC valve is duty controlled.
In many of the conventional systems in which the ISC valve is duty controlled, a drive frequency of the duty control is set relatively low so as to obtain a high level of reliability. Accordingly, the drive frequency of the duty control is generally sufficiently lower than a characteristic frequency of the intake passage. However, a recent trend has been such that the drive frequency of the duty control is set to a given high level so as to improve the accuracy and responsiveness of the idle speed control particularly in engines having a large cubic capacity. Thus, the drive frequency of the duty control may approximate to the characteristic frequency of the intake passage between the air cleaner and the throttle valve. In this case, the vibration accompanying an intermittent air flow caused by the ISC valve duty controlled in an idle speed region is amplified due to the resonance in the intake passage, thereby being transmitted to the airflow meter. Influenced by the above vibration, an output of the airflow meter may carry an error particularly when the highly sensitive air flow meter such as a hot-wire type, is used.